EPR studies have been undertaken at liquid-helium temperatures have been undertaken to detect in the mammalian respiratory chain possible high-energy forms of cytochromes involved in energy coupling at the three sites of oxidative phosphorylation. Possible model systems bearing on the mechanism of energy conservation are also being investigated. Two of these systems include ferricytochrome c3-a multi-heme protein with properties similar to mammalian cytochrome oxidase which is implicated at site III and the respiratory chain of Azotobacter vinelandii which is very similar in catalytic activities and respiratory components to the mammalian respiratory chain. EDP studies have indicated a new type of respiratory component which unusual EPR parameters indicating the interaction between iron atoms of respiratory components is far more complex than previously thought. A further understanding of these components is essential to both the mechanism of electron transfer and thus of energy conservation.